The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition engines function by introducing a fuel/air mixture into a combustion chamber and igniting the mixture using an ignition source such as a spark plug. A spark-ignition engine can operate at a rich air/fuel ratio, an air/fuel ratio at or near stoichiometry, and at a lean air/fuel ratio. A spark-ignition engine operates at the lean air/fuel ratio, including operating in a stratified charge mode which includes operating substantially un-throttled with fuel directly injected in each combustion chamber during a compression stroke, just prior to initiation of spark. It is known to transition a spark-ignition engine from a stratified charge combustion mode to a homogeneous charge combustion mode to effect regeneration of a lean-NOx reduction catalyst. Operating a spark-ignition engine in a homogeneous charge combustion mode includes operating at stoichiometric air/fuel ratio, with an engine throttle valve controlled to a predetermined position, and with fuel directly injected in each combustion chamber during an intake stroke prior to the compression stroke and spark ignition.
Known aftertreatment systems for spark-ignition engines operating lean of stoichiometry can include a lean-NOx reduction catalyst, which can be used in concert with other exhaust aftertreatment devices including three-way catalytic converters. A lean-NOx reduction catalyst operates most effectively within a temperature range that is narrower than an exhaust gas feedstream temperature range occurring during engine operation that includes engine starting and high-speed, high-load operation. A lean-NOx reduction catalyst requires periodic regeneration to desorb and reduce adsorbed NOx elements. Known regenerative techniques include operating the spark-ignition engine at an air/fuel ratio that is at stoichiometry or rich of stoichiometry.